That has seen almost $600 million wiped off the value of Forrest’s stake in Fortescue, which has dropped from $6.096 billion at the start of January to $5.5 billion based on Wednesday’s closing price of $5.35.

Not only are prices at six-month lows, but some analysts including Goldman Sachs have forecast the iron ore price could drop as low as $US80 a tonne this year as new supply comes on line.

The $US100-a-tonne mark is seen as crucial for smaller iron ore miners; below this, profitability is more difficult.

Of course, Forrest is well used to the gyrations of the sharemarket causing big fluctuations in his net worth. In the last 12 months, Fortescue’s share price has dipped as low as $2.87, at which point Forrest’s stake was worth about $3 billion.